PGP and leukemia: Drug resistance in HL60R variant cells is characterized by overexpression of a variety of proteins, which belong to the P-gp but also by constitutive expression of inhibitor of apoptosis proteins (IAPs), family members proteins that we previously described play an important role in tumor cell resistance to drug induced apoptosis and show also a different expression in drug-resistant and -sensitive HL60 leukemia cells [35,36].